Beginning SQL

Executing the preceding SQL creates the following results set:
SalesPersonId FirstName LastName MemberId FirstName LastName Repeat Orders
1 Sandra Hugson 4 Steve Gee 3
1 Sandra Hugson 6 Jenny Jones 3
1 Sandra Hugson 7 John Jackson 1
1 Sandra Hugson 9 Seymour Botts 6
2 Frasier Crane 4 Steve Gee 3
2 Frasier Crane 7 John Jackson 3
2 Frasier Crane 10 Susie Simons 1
As you would expect, these results are identical to the previous SQL’s results. So which query is best?
First, it depends on which query you find easiest to use. Programmer time is often more expensive than
computer time, so if one way of writing a query is easier for you, then stick with it unless there’s a good
reason to do it another way.
The other side of things is efficiency — which query runs faster? In the two different ways shown, the
first is slightly more efficient. It has one less join than the second way, because the second way has to
have an INNER JOIN of the Orders table to itself. The first way gets the same results but filters out
nonduplicate orders using a HAVING clause.
You may even be able to think of a third or fourth (or fifth or sixth) way of writing the same query, one
that might be even faster. In the real world, however, most programmers don’t have the time to write the
same code in five different ways in a bid to save fractions of a second. Unless, of course, efficiency is
absolutely paramount — for example, where there are millions and millions of records and your initial
SQL runs with all the speed of a tortoise through molasses.
To help you speed up your queries, the next section covers some tips on writing efficient queries.
Top Tips for Efficient Queries
❑ When using AND, put the condition least likely to be true first. The database system evaluates
conditions from left to right, subject to operator precedence. If you have two or more AND operators
in a condition, the one to the left is evaluated first, and if and only if it’s true is the next condition
evaluated. Finally, if that condition is true, then the third condition is evaluated. You can
save the database system work, and hence increase speed, by putting the least likely condition
first. For example, if you were looking for all members living in New State and born before
January 1, 1940, you could write the following query:
SELECT FirstName, LastName
FROM MemberDetails
WHERE State = ‘New State’ AND DateOfBirth < ‘1940-01-01’;
Advanced Queries
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